It is known to associate shear-action mixing devices with worms or extruder units so that the shear action of the mixer can contribute to the homogeneity of a thermoplastic synthetic resin which emerges from the mixer.
A mixing apparatus for thermoplastified synthetic resins or thermoplastifiable synthetic resins can comprise an elongated stator receiving an elongated rotor which is rotatable about a common axis of the stator and rotor and which together define a gap, referred to herein as the shear gap. To promote the mixing action, the outer periphery of the rotor and the inner periphery of the stator can be provided with circumferential rows of angularly equispaced mixing chambers in the form of recesses opening at the respective periphery and also lying in axially spaced relationship in axial rows.
The recesses of the stator and the rotor may have similar outlines and can be elongated in the axial direction. Each recess may have a pair of opposite ends, referred to as a front end and a rear end, respectively, which may be semicircular in shape and are defined by a guide radius (R).
It is known to provide the numbers of mixing chambers in the corresponding peripheral rows of the rotor and stator so that they are the same and the number of mixing chambers in the axially extending rows to correspond to the number of mixing chambers of the axially extending rows of the rotor, and to offset the peripheral rows of mixing chambers of the rotor with respect to the peripheral rows of the stator by about half the length of the mixing chambers.
Apparatus of this type is commonly mounted on a worm-type extrusion press in which the thermoplastic synthetic resin is plastified, to further homogenize this material at the outlet of the worm press, in which case the rotor can be connected to and driven with the worm, or can be provided as an intermediate stage or section along the length of the worm extrusion press.
The stator can be constituted as rings or sleeves (bushings) which can be received in a cylinder.
Reference may be made herein to mixing-chamber recesses which are identical. While, for the most part, the mixing-chamber recesses, especially of the rotor but also of the stator, will be formed in a cylindrical wall of the respective part, generally at the ends of the rotor, the latter may deviate from the cylindrical, thereby requiring some difference between the mixing-chamber recesses at such ends from the mixing-chamber recesses along the remainder of the body of the rotor.
It thus is within the framework of the invention to allow front and end mixing-chamber recesses to deviate in configuration and dimensions in accordance with the geometry of the rotor or stator, especially when the rotor or stator has frustoconical portions at one or the other end, forming transition pieces between the rotor and the extruder worm.
In a known mixer construction of this type (see WO 85/91911), the mixing chambers of the rotor are deeper than those of the stator. In a radial cross section, these mixing-chamber recesses are arcuate but there is no specific relationship between the radius of curvature of the cross section of the recess to the guide radius previously mentioned.
In practice, the mixing efficiency, defined as the quotient between the homogenization achieved and the energy used, is generally unsatisfactory. The extrudate shows unmixed or poorly mixed cells.
The problem is even more pronounced when the device is used for mixing additives into the thermoplastic synthetic resin.
Other configurations of the mixing chambers have been used as well. For example, in the U.K. patent 930 339, the mixing chambers have a rectangular to trapezoidal configuration in radial section.
European patent EP 00 48 590 suggests that the mixing chambers should be hemispherical in configuration, with the mixing chambers not only of the rotor and stator being offset, but being offset as well along the rotor and the stator, respectively.
The increase in offsetting and the hemispherical configuration of the mixing chambers does improve the mixing, but only at the expense of an increased energy consumption.